Spark gap



Sept. 18, 1956 w. R. BAKER SPARK GAP Filed July 14, 19,54

OSCILLATOR r0 m I I 43 4 44 g I I {V 0.0. 1 POWER 3 SUPPLY LOAD 62INVENTOR.

7 WILLIAM R. BAKER BY AMPLIFIER r 01 63 ATTORNEY.

Fa-tented Sept. 18, 1956 ice SPARK GAP William R. Baker, Berkeley,Calif., assignor to the United States of America as represented by theUnited States Atomic Energy Commission Application July 14, 1954, SerialNo. 443,448 Claims. (Cl. 317-31) The present invention relates to aspark gap and, more particularly, to an electrical protective device andcircuit for a radio-frequency transmission line.

It has been found that known protective devices, such as spark gapsmounted within gas-filled envelopes, are inadequate in the high voltage,high-frequency field because -the operation thereof is too slow and,further, such devices cannot withstand the overload conditions whichoccur. Also, the known types of such devices generally become a part ofthe system to be protected and add undesirable factors which may alterthe wave form of the voltage to an extent that corrective measures arenecessary. The required corrective measures often lower the etficiencyof the system.

The present invention overcomes the foregoing disadvantages by providinga protective device which is connected into the system at a minimumvoltage node. A direct current control voltage having a value below thecontinuous discharge value for the device is impressed between twospaced-apart electrodes. It has been found that a fault results in achange of the voltage at the node so that a small increase in theradio-frequency voltage at the protective device results in a lowresistance path for the direct current to spark over. The surge in thedirect current is then utilized to disable the radio-frequency powerconnection to the system.

It is therefore an object of the invention to provide a new and improvedspark gap device.

Another object of the invention is to provide a high speed protectivesystem for .a radio-frequency transmission line circuit.

A further object of the invention is to provide a spark gap device to beconnected into a radio-frequency transmission line at a minimum voltagenode of the standing wave of voltage.

A still further object of the invention is to provide a low-resistancepath fora .control current upon a fault in the radio-frequency circuit.

Still another object of the invention is to provide a spark-gap devicehaving an impressed control voltage below the spark level forinstallation at a minimum node on a radio-frequency transmission line.

Other objects and advantages of the invention will be apparent in thefollowing description and claims .considered together with theaccompanying drawing, in which:

Figure 1 is a view of the spark gap, partly in section, as installed;and

Figure 2 is a schematic wiring diagram of a circuit incorporating thespark gap of Fig. 1.

Referring to the drawing in detail, Fig. 1 in particular, there isprovided a coaxial transmission line 11, illustrated partially and incross section. Such transmission line 11 has a center conductor 12 andan outer conductor 13, the latterhaving an aperture 14. Amushroom-shaped first electrode 15 is suitably mounted, as by a screw 16on the center conductor 12, in alignment with the aperture 14.

Centrally disposed within the aperture 14 is a hollow second electrode17 having a semi-spherical portion 18 extended toward and spaced fromthe first electrode. To maintain the position of the second electrode 17such electrode is suitably mounted within an open-ended housing 21 whichengages the outer conductor 13 as by threads 22. It is to be noted thata good electrical connection should be provided between the housing 21and the second electrode 17 so that the latter provides electricalcontinuation of the former.

A third rod electrode 26 is disposed coaxially within the hollow secondelectrode 17 and extends through an opening 27 in the semi-sphericalportion 18 of the latter. It is to be noted that the opening 27 shouldbe larger than the third electrode 26 to provide insulating spacetherebetween and that the extended end of the third electrode isrounded. The mounting of the third electrode 26 in the second electrode17 may be accomplished in any conventional manner to provide suitableinsulation between the two electrodes. A conductor 28 is connected atone end to the third electrode 26 and is extended transversely thereofthrough an aperture 29 in the second electrode 17 to an electricalreceptacle 31 suitably mounted in the wall of the housing 21.

In the illustration of the drawing, see Fig. 2, there is provided aconventional high-power and high-frequency oscillator 36 having a powerinput 37 connected to a suitable source 38 of energizing power. One ofthe connections 39 between the source 38 and the input 37 includes anormally closed contactor 41 of a conventional high speed relay 42. Oneof the output terminals 43 of the oscillator 36 is connected to groundand the other terminal 44 is connected to the center conductor 12 of thetransmission line 11. Thus, by connecting the outer conductor 13 of theline 11 to ground and connecting the center conductor 12 at the extendedend of the line to a coupling loop 45, which extends into a cavityresonator load 46 and is grounded at a wall of the resonator, power istransmitted from the oscillator 36 to the load 46. At a point on thetransmission line 11 Where the standing wave of the radio-frequencyvoltage is at a minimum node, as indicated by the diagram 51 of Fig. 2,the three electrodes 15, 17, and 26 of the spark gap arrangement areinstalled in accordance with the foregoing description.

Now, in accordance with the invention, there is provided a conventionaldirect current power supply 56 having a grounded positive terminal 57and a negative terminal 58. A radio-frequency choke coil 61 is connectedfrom the negative terminal 58 to the third electrode 26 of the spark gapto protect the power supply 56 from voltage surges and to prevent theradio-frequency voltage of the transmission line from affectingthe'power supply. Voltage variations at the junction between the chokecoil 61 and third electrode 26 are coupled to the input of an amplifier62 by a capacitor 63 connected therebetween. The coil of the relay 42 isconnected across the output of the amplifier 62. Thus, it is seen thatthe voltage of the power supply 56 appears between the first and thirdelectrodes 15 and 26 of the spark gap and also between the second andthird electrodes 17, 2.6 so that any current flowing between such pairof electrodes will develop a voltage at the input of the amplifier 62which, in turn, results in operation of the relay 42 to open the inputcircuit or" the oscillator 36.

Having thus described the elements of the invention in detail, theoperation will now be described. With the direct current power supply 56suitably energized, a unidirectional voltage appears between the firstand third electrodes 15, 26 and between the second and third electrodes17, 26. Proper adjustment of the gap between such electrodes 15, 26 andof the value of the unidirectional voltage applied will prevent a flowof direct current.

The contactor 41 of the relay 42 remains in a closed condition whenthere is no excitation of the coil. Thus, the output voltage of theradio-frequency oscillator 36 appears at the input end of thetransmission line 11. A standing wave of voltage appears along thetransmission line 11 with a minimum voltage node occurring at theposition of the spark gap elements. As long as the voltage at such noderemains at such minimum value, or lower, there will be no direct currentflow from the power supply 56.

Now, upon the occurrence of a fault in the radio-frequency circuit thestanding wave voltage distribution is disturbed so that an increasingvoltage appears at the former minimum voltage node position. Because ofthe small gap between the first and second electrodes 15, 17, theincreasing voltage therebetween soon reaches the breakdown value of thegap and a spark occurs. T he are thus established by the radio-frequencyvoltage provides a low-resistance path for the direct current betweenthe first and third electrodes 15, 26. The flow of direct currentbetween such electrodes 15, 26 results in a voltage drop across thechoke coil 61 and in a voltage change at the junction of the coil andthird electrode being coupled to the input of the amplifier 62. Theoutput of the amplifier 62 then energizes the coil of the relay 42 toopen the contactor 41. The input to the radio-frequency oscillator 36 isthen broken and the radio-frequency voltage of the transmission line 11becomes zero.

The foregoing operation at the time of a radio-frequency fault issubstantially instantaneous (in the order of a few microseconds) withthe occurrence of the fault. While the relay 42 has been illustrated inthe output circuit of the amplifier 62, such relay could readily beconnected to operate in the connection between the third electrode 26and the terminal 58 of the power supply 56.

It is to be noted that the spark gap device of the present inventioncomprises two coaxial electrodes 17, 26 facing a larger electrode 15,thus providing an air gap between the coaxial electrodes as well as twoseparate voltage gaps between the electrodes 15, 17 and 15, 26. The airgap between the coaxial electrodes 17, 26 separates the spark due toradio-frequency current and the spark due to direct current so as toreduce the intensity of the are over the intensity where only twoelectrodes are used. Also, the duration of the spark is reduced. Suchadvantages result from the fact that once a spark-over to the firstelectrode 15 has occurred there is suificient ionization in the gapbetween the second and third electrodes 17 and .26 to cause a secondaryspark which short-circuits the direct current power supply 56 therebyextinguishing the spark between the first and third electrodes 15, 26because of the direct power supply to provide protection from sparkingdamage to the transmission line.

It has been found, for example, that for a radio-frequency voltage rangefrom IGKV to 20K! suitable spark gap settings between 7 inch and /2 inchare satisfactory. With such gap settings the direct current bias betweenthe first and third electrodes 15, 26 may be established between 800volts and 1000 volts for satisfactory operation.

While the salient features of the present invention have been describedin detail with respect to a single embodiment, it will be apparent thatnumerous modifications may be made within the spirit and scope of theinvention,

and it is therefore not desired to limit the invention to the exactdetails shown except insofar as they may be defined in the followingclaims.

What is claimed is:

1. In a spark gap device for installation on a two-conductortransmission line having a high-frequency standing wave voltagedistribution, the combination comprising a first electrode mounted onone conductor of said line at a voltage node, a second hollow electrodemounted on the other conductor of said line in alignment with said firstelectrode, said first and second electrodes spaced apart a predetermineddistance, a third electrode mounted coaxially within said secondelectrode and extended through an opening therein, and means connectedbetween said one conductor and said third electrode and between saidsecond and third electrodes for applying a direct current having a valuebelow the continuous discharge value.

2. In an electrical protective device, the combination comprising afirst and second electrode disposed in spacedapart relation andrespectively connected across a source of high voltage having a highfrequency, a third electrode mounted coaxially within said secondelectrode and extended through an opening therein, and direct currentmeans connected between said first and third electrodes and between saidsecond and third electrodes.

3. In an electrical protective. device, the combination comprising afirst and second electrode disposed in alignment and in spaced apartrelation, a high voltage and high-frequency source respectivelyconnected to said first and second electrodes, a third electrodedisposed coaxially within said second electrode in insulated relationand having an end thereof extended through an opening in said secondelectrode, and direct current means connected between said first andsaid second electrodes and between said second and third electrodes.

4. The combination of claim 3 wherein said first electrode is spacedapart from said second and third electrodes by a gap length greater thanthe continuous discharge gap length at the values of high frequency anddirect current voltages.

5. In a spark gap device for installation on a coaxial transmission linesystem having a high-frequency standing wave voltage distribution, thecombination comprising a button-type first electrode disposed inelectrical contact on the center conductor of said coaxial transmissionline at a voltage node of said standing'wave voltage, a hollow andtubular second electrode disposed through and in electrical contact withthe outer conductor of said coaxial transmission line and aligned withsaid first electrode, an elongated third electrode mounted coaxially inand insulated from said second electrode with one end extended throughan opening in said second electrode toward said first electrode, and adirect current source connected between said first and second electrodesand between said second and third electrodes.

References Cited in the file of this patent UNITED STATES PATENTS1,477,305 Allcutt Dec. 11, 1923 7 2,366,660 Usselman Ian. 2, 19452,498,720 Wild Feb. 28, 1950

